tools/nolibc/printf: Prepend sign to converted number

tools/nolibc/printf: Prepend sign to converted number

Instead of appending the converted number to the sign, convert first
and then prepend the sign (or "0x").
Use the length returned by u64toh_r() instead of calling strlen().

Needed so that zero padding can be inserted between the sign and digits
in an upcoming patch.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260308113742.12649-12-david.laight.linux@gmail.com
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>